The tour of the footplate finished, Simnel turned to Moist. ‘According to our schedule, we ought to be going.’ He blew his whistle and shouted down the platform, ‘All aboard!’

There was no way of avoiding considerable attention as the King’s party assembled to board the armoured carriages behind Iron Girder. The engine herself was spectacular and her passengers were unusual even for a city like Zemphis. There were the dwarfs: the Low King and his bodyguards, Aeron his secretary and Bashfull Bashfullsson; there were suspiciously deep shadows that suggested the continued presence of dark clerks; there were several specialist members of the Watch;^[73] there were the goblins, who scrambled into the guard’s van being coupled on at the rear; and just in front of the van was a flatbed carrying Constable Bluejohn along with equipment and baggage too bulky for the guard’s van.

Iron Girder was fully in steam again now and the vapour surrounded everybody. Vimes went with the engineers as they walked right around the train for a final check. Then came the scream of the whistle and the little shunting dance that Iron Girder did before gaining speed. With someone staring from every carriage window, the special express train to Uberwald started to show what it could do.

The story told locally about the Paps of Scilla was that they were formed when one huge mountain fell apart, leaving a treacherous network of broken caverns – some full of water, always overflowing – topped by the eight forbidding peaks, which seemed to hang in the moisture-laden air, surrounded by rainbows. After the boomerang incident in Zemphis, Moist wasn’t very keen on seeing the Paps up close and personal, but Simnel’s surveyors had excelled themselves. The railway track insinuated itself up through the craggy gaps so that the train climbed majestically higher and higher, leaving the city of Zemphis and the heat shimmer of the sierra far below.

Halfway towards the gloom of the pass between the tallest peaks, the train emerged from a large natural tunnel into another kaleidoscope of rainbows, which were distracting even when people weren’t throwing things at your head.

Without warning, a boulder smashed down in front of the train and rolled across the tracks, to shatter in the opposite gully. Then there was another crash from the rear. The train shuddered horribly, and carried on.

Moist looked up and saw dwarfs perched on the craggy cliffs on each side of the canyon, levering boulders down on to the train. Commander Vimes could be heard cursing and shouting orders down the carriages, his words drowned out as yet larger boulders fell, raining down on the locomotive, which was moving slowly forward like an old lady testing the water.

Surely, Moist thought, this is the end. Even if the tracks ahead remain undamaged, no normal engine could withstand this bombardment. But then he realized that Iron Girder, slowly and methodically, was actually steaming on despite the boulders continuing to pound the train.

Moist couldn’t stop himself. He shouted to anyone who would listen, ‘They’re bouncing off! It’s the sorortanium, it takes the punishment and throws it right back!’